POLYPHOSPHATE GRANULES ARE AN ARTIFACT OF SPECIMEN PREPARATION IN THE ECTOMYCORRHIZAL FUNGUS PISOLITHUS-TINCTORIUS

Polyphosphate granules are precipitated in the vacuoles of the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch by various treatments, including conventional specimen preparation. Granules are not produced by glutaraldehyde fixation but appear at early stages of ethanol dehydration and are visible with Nomarski DIC microscopy. They show gamma-metachromasy with toluidine blue O at low pH, are extracted by cold trichloroacetic acid and contain phosphorus and calcium as demonstrated by X-ray microanalysis. The granules are surrounded by electron-lucent areas that do not contain these elements at detectable levels. In contrast, vacuoles of freeze-substituted hyphae contain evenly dispersed flocculent material. Phosphorus and potassium are distributed more or less uniformly throughout, but calcium is not detected. This indicates that polyphosphate is present in the vacuole of living hyphae in soluble form and is precipitated to form granules by various treatments. It is thought that granules form when membranes, including the tonoplast, become leaky and there is an influx of precipitating ions such as calcium.